The United States and Japan will step up their defence cooperation to deal with the threat from nuclear-armed North Korea as tensions in East Asia remain high, officials from the two allies said on Thursday.

Gallery defends removing Joko portrait

A photograph of Indonesian President Joko Widodo has been removed from the walls of the National Portrait Gallery for fear it would be damaged.

Director Angus Trumble said it was a pre-emptive and temporary measure, following a negative reaction to the portrait by some visitors.

"In view of the circumstances ... and my best assessment of the risk of damage to the work of art, it was necessary to remove it from public display", he said in a statement on Thursday, noting the disapproval of the artist Adam Ferguson.

Mr Ferguson, who is reportedly in Nepal at the moment, said he was perplexed by the decision.

He said he would have thought that allowing people to engage with his work now would be more important given the circumstances.

"I am totally perplexed that a leading Australian artistic institution would take such action," he wrote on his Facebook page.